Styrene-based thermoplastic elastomers have rubber elasticity at room temperature, excellent physical properties such as flexibility and moldability, low specific gravity, and excellent recyclability. Therefore, while being accompanied by problems such as environmental pollution, the styrene-based thermoplastic elastomers have recently been used as a substitute for vulcanized rubber and polyvinyl chloride in a wide range of fields including automobile parts, industrial parts, sundry goods, and sporting goods.
Among the styrene-based thermoplastic elastomers, a styrene-butadiene-styrene block copolymer (SBS), a styrene-isoprene-styrene block copolymer (SIS), and hydrogenated products thereof have been widely used due to their low cost as well as excellent flexibility, rubber elasticity, recyclability, and the like.
Meanwhile studies have been conducted to improve various physical properties of styrene-based thermoplastic elastomer compositions, and the following compositions, for example, have been proposed:
1) a composition having a good balance of heat resistance and flow characteristics, which includes a polyolefin-based resin, and a hydrogenated product of a block copolymer having a polymer block mainly containing α-methylstyrene and a polymer block containing isoprene and/or butadiene (see PTL 1);
2) a composition having excellent scratch resistance and abrasion resistance, which includes an acryl-based polymer, a hydrogenated product of a block copolymer including a polymer block mainly containing α-methylstyrene and a polymer block containing isoprene and/or butadiene, and a softener (see PTL 2);
3) a thermoplastic resin composition having excellent moldability and abrasion resistance, which is a thermoplastic resin composition for a golf ball, including a styrene-based thermoplastic elastomer, a softener, a peroxide-crosslinkable olefin-based resin, a peroxide-decomposable olefin-based resin, and an organic peroxide, and (see PTL 3); and
4) a composition having excellent releasability and abrasion resistance in powder molding, which is a resin composition for powder molding, including a polypropylene-based resin, an olefin-based thermoplastic elastomer, a styrene-based thermoplastic elastomer, and a silicone oil-containing olefin-based resin (see PTL 4).
However, the composition of PTL 1 has excellent heat resistance and flow characteristics, but it is hard to say that the composition has sufficient abrasion resistance. Further, the composition of PTL 2 is highly flexible and has excellent properties such as molding processability and transparency while maintaining surface characteristics such as surface hardness, weather resistance, and transparency, attributed to the acrylic-based resins. However, since the acrylic-based resins have hygroscopicity, a drying step or the like is required at a time of molding processing, and as a result, the molding processing step is complicated. PTL 3 describes Taber abrasion values with regard to the composition of PTL 3, but the values are not fully satisfactory. Further, the composition requires dynamic cross-linking, and accordingly, the molding processability is deteriorated. In addition, with regard to the composition of PTL 4, the evaluation results of the properties of the molded surfaces before and after an abrasion test using an unbleached muslin No. 3 cloth are disclosed, but the results of abrasion resistance evaluation are not fully satisfactory. In addition, the silicone oil or the silicone rubber is problematic in bleeding on the molded surface.
Therefore, the present inventors have once developed the following composition in order to provide a composition having good flexibility, lightweightness, molding processability, and heat resistance, in particular, excellent abrasion resistance (see PTL 5).
5) a thermoplastic polymer composition including at least one block copolymer (a) selected from an α-methylstyrene-based block copolymer with a number average molecular weight of 30,000 to 500,000, including a polymer block A containing α-methylstyrene units and a polymer block B containing conjugated diene compound units, and a hydrogenated product thereof; a propylene-based polymer (b); and an ethylene-based polymer (c) having a density of 0.94 g/cm3 or less, at a ratio satisfying the following expressions (1) and (2):0.1≦W(a)/[W(a)+W(b)+W(c)]≦0.81≦W(b)/W(c)
wherein W(a), W(b), and W(c) represent masses of the block copolymer (a), the propylene-based polymer (b), and the ethylene-based polymer (c), respectively.
In addition, as a composition having excellent fluidity, tensile strength at break, and weld strength:
6) a thermoplastic polymer composition including a block copolymer and/or a hydrogenated product thereof (a) having a polymer block A mainly containing α-methylstyrene units and a polymer block B mainly containing conjugated diene compound units; and a hydrogenated product (b) of a block copolymer having a polymer block A′ mainly containing aromatic vinyl compound units other than α-methylstyrene units and a polymer block B′ mainly containing conjugated diene compound units, in which the number average molecular weight of the block copolymer and/or a hydrogenated product thereof (a) is 30,000 to 350,000, and the hydrogenated product (b) of the block copolymer is 100,000 to 500,000 has been proposed (see PTL 6). Further, as a composition for use in an elastic band, which has a good balance of flexibility, tensile strength, permanent tensile strain, and heat resistance,
7) a thermoplastic elastomer composition including a block copolymer (a) with a number average molecular weight of 130,000 to 600,000, obtained by hydrogenation of a block copolymer including a polymer block mainly containing styrene units and a polymer block mainly containing conjugated diene compound units; and a block copolymer (b) with a number average molecular weight of 70,000 to 600,000, obtained by hydrogenation of a block copolymer including a polymer block mainly containing α-methylstyrene units and a polymer block mainly containing conjugated diene compound units, in which the mass ratio of (a):(b) is 2:98 to 98:2 and the hardness is 1 to 90
has been proposed (see PTL 7).